Bar chart or stacked bar for survey responses?
Diverging stacked bars centred between agree and disagree, because the question people ask is which way opinion leans. Plain stacked bars force the reader to compare segment lengths that do not share a baseline.
I keep hearing p-values are misused. What do they mean?
The probability of data at least this extreme if the null hypothesis were true. That is all. It is not the probability the hypothesis is false, and a small p with a tiny effect size usually means a large sample rather than an important finding.
How many users do I need for an A/B test?
It depends on the effect you would act on, not the effect you hope for. Decide the smallest difference worth shipping, then compute the size for that. Running until significance appears is the most common way to get a result that will not replicate.